Whether the normal immune system can prevent or control carcinogenesis, i.e., the development of cancer, is central to mechanisms of homeostasis, surveillance, prevention, and interventive management of neoplastic, mutated, infected, or otherwise altered cells. Naturally occurring lymphocytes, macrophages, and other leukocytes and their secretory products, e.g., lymphokines, interleukins and other immunologic hormones, are being studied to define their effective anticarcinogenic and tumor cell growth inhibitory activities. Lymphotoxin, one of the few lymphokines with cytotoxic activity can prevent carcinogenesis and inhibit tumor cell growth. Anticarcinogenic action is direct and irreversible. Inhibition of tumor cell growth is primarily reversible but can become irreversible due to increased susceptibility of preneoplastic and neoplastic cells to cytolytic destruction by natural killer cells and by macrophages resulting rom lymphotoxin-target cell interaction. The direct acting anticarcinogenic activity of lymphotoxin is more potent than the tumor cell inhibitory activity but through indirect mechanisms the lymphokine may be an effective homeostatic mechanism for control of carcinogenesis at its later stages of development.